SPRINGFIELD (Illinois News Network) – State health officials are investigating dozens of food poisoning cases in Illinois possibly linked to eating salad at McDonald’s, but the state’s top doctor said it may be more widespread.

The Illinois Department of Public Health is investigating about 90 cases of cyclosporiasis, an intestinal illness caused by the Cyclospora parasite. The foodborne illness has been reported in counties across Illinois with people becoming ill starting in mid-May.

“Roughly a quarter of those cases in Illinois have reported eating a salad that was produced for McDonalds in the days before they started becoming ill,” said IDPH director Dr. Nirav Shah.

McDonald’s stopped selling the salads as a precaution.

“McDonald’s has been in contact with public health authorities from Iowa and Illinois about an increase in Cyclospora infections in those states,” a statement on the company’s website said. “Out of an abundance of caution, we decided to voluntarily stop selling salads at impacted restaurants until we can switch to another lettuce blend supplier. We are in the process of removing existing lettuce blend from identified restaurants and distribution centers – which includes approximately 3,000 of our U.S. restaurants primarily located in the Midwest. McDonald’s is committed to the highest standards of food safety and quality control. We are closely monitoring this situation and cooperating with state and federal public health authorities as they further investigate.”

Cyclospora poisoning exhibits symptoms similar to other types food poisoning.

“Frequent bouts of watery diarrhea, fatigue and, to a lesser extent, nausea and vomiting are the classic symptoms,” Shah said.

He said that health officials also are investigating other sources because only a quarter of the people who fell ill ate at McDonalds.

Epidemiologists and others at IDPH must go interview all of the confirmed Cyclospora cases, Shah said, then find any common links between them.

“That’s how we identify these clusters,” Shah said. “It’s one part science, one part art, and one part vigilance.”